1. Field of the Invention
The present invention relates to a telephoto lens system, and more particularly to an improvement of an internal focusing type telephoto lens system.
2. Description of the Prior Art
In the field of this art, it has been well known to provide a photographic lens system in which the whole lens system is shiftable along the optical axis for focusing thereof. This is sometimes a relatively easy expedient for focusing in smaller compact lens system of a fixed focal length. However, in a telephoto type lens system, its diameter, overall length and weight are by necessity increased over both a standard type or wide angle type lens system. Thus, any focusing arrangement which requires the shifting of the entire telephoto lens system would also requires mechanisms that are by necessity relatively complex, strong and durable. Additionally, the shifting distance for focusing is also necessarily increased. Therefore, a focusing arrangement which requires the shifting of the whole lens system would not be favorable for a telephoto lens system.
Thus, in the field of telephoto lens systems, there have been provided various designs in which a part of the whole lens system alone is shiftable for focusing thereof. For example, Japanese Laid-Open Patent No. SHO 56-165110 discloses a telephoto lens system in which an internal lens unit is shiftable for focusing. The focusing arrangement of this internal focusing type would require mechanisms that are compact and light-weight in comparison with mechanisms required for shifting of the whole lens system. In the above-mentioned Japanese patent, the embodiment discloses a telephoto lens system of an internal focusing type which comprises, from the object side to the image side, a first positive lens unit, a second negative lens unit, a third negative lens unit, and a fourth positive lens unit, the third lens unit alone being shiftable along the optical axis for focusing with other lens units being stationary. The lens system of the above-mentioned embodiment has the F-number of 1:2.8, and the field angle of 12.3.degree. which corresponds to the focal length of 200 mm with respect to 35 mm SLR cameras.